1. Field of the Invention
The present invention relates to a surface acoustic wave device having excellent balancing characteristics between balanced terminals and being suitable as filters in duplexers of communication devices, etc., and the present invention also relates to a communication device including the surface acoustic wave device.
2. Description of the Related Art
In recent years, technical advances in reducing the size and weight of portable telephones have been remarkable. To realize these, not only reduction in the number of parts and their size, but also development of multifunctional parts is in progress. Under such conditions, surface acoustic wave devices having a filtering function which are used in the RF stage of portable telephones, the surface acoustic wave devices also having a balanced-to-unbalanced conversion function or a so-called balun function, have been intensively studied in recent years and have come to be used primarily in GSM (Global System for Mobile communications) devices, etc. Furthermore, surface acoustic wave devices have begun to be used in AMPS, PCS, DCS, etc., and it is considered that the demand for surface acoustic wave devices having such a balanced-to-unbalanced conversion function will further increase.
In surface acoustic wave filters to be used in surface acoustic wave devices having a balanced-to-unbalanced conversion function, the construction shown in FIG. 27 is widely used. In the construction in FIG. 27, longitudinally-coupled surface acoustic wave filter elements 101 and 102 are mounted on a piezoelectric substrate 118 such that terminals of each are electrically connected in parallel and that the other terminals of each are connected in series. The surface acoustic wave filter element 101 is provided with three comb-shaped electrode portions (interdigital transducers, hereinafter referred to as IDTs) 104, 103, and 105 and is also provided with reflectors 106 and 107 so as to sandwich the IDTs 104, 103, and 105. The surface acoustic wave filter element 102 is provided with three IDTs 109, 108, and 110 along the propagation direction of a surface acoustic wave and is also provided with reflectors 111 and 112 so as to sandwich the IDTs 109, 108, and 110.
The difference between the surface acoustic wave filter element 101 and the surface acoustic wave filter element 102 is that the IDT 103 is opposite in polarity to the IDT 108. Thus, the signals output from terminals 114 and 115 are 180 degrees out of phase from each other, and an unbalanced signal input from a terminal 113 is converted and output as a balanced signal between the terminals 114 and 115.
In a surface acoustic wave filter having a balanced-to-unbalanced conversion function, regarding the transmission characteristics in the passband between the unbalanced terminal and each of the balanced terminals, it is required that the amplitude characteristics be equal and the phase difference be 180 degrees. Furthermore, outside the passband, both the amplitude characteristics and the phase characteristics should be equal.
Regarding the amplitude balancing and phase balancing, when the above-described surface acoustic wave device having a balanced-to-unbalanced conversion function is considered to be a three-port device in which, for example, the unbalanced input terminal is assumed to be a first port and the balanced output terminals are assumed to be a second port and third port, respectively, the amplitude balancing and phase balancing are defined as follows:Amplitude balancing=|A|, A=|20 log(S21)|−|20 log(S31)|phase balancing=|B−180|  (1)wherein B=|∠S21−∠S31|.
Here, when a three-port device is expressed as a scattering matrix, S21 and S31 represent components of the matrix and show the transmission characteristics between port 2 and port 1 and between port 3 and port 1, respectively. Regarding such balancing, it is accepted that ideally the amplitude balancing is 0 dB and the phase balancing is zero degree inside the passband of the filter, and the amplitude balancing is 0 dB and the phase balancing is 180 degrees outside the passband.
As shown in FIGS. 28 and 29, in a related surface acoustic wave filter 100 having a balanced-to-unbalanced conversion function, a piezoelectric substrate 118 is fixed to a packaging member 120 by an adhesive layer 122. Furthermore, the packaging member 120 is made of, for example, dielectric ceramics such as alumina, etc. In such a surface acoustic wave device, as shown in FIG. 29, the surface acoustic wave filter 100 is housed inside the packaging member 120 by using the adhesive layer 122 and is electrically connected to the packaging member 120 through a bonding wire 123, and then sealed by using a cap 124 as a cover.
When the surface acoustic wave filter 100 having a balanced-to-unbalanced conversion function is die bonded (fixed) to the packaging member 120 by using the adhesive layer 122, coating locations of the adhesive layer 122 vary and mounting locations of the surface acoustic wave filter 100 vary in the packaging member 120.
Therefore, in related products, there is a problem in that capacitances between electrodes and lead wires of the IDTs on the top surface of the surface acoustic wave filter 100 and metalized patterns (electrode terminals) on the packaging member 120 vary and that the balancing, the phase balancing in particular, greatly varies. Moreover, such variations of the balancing also occur in surface acoustic wave filters with balanced inputting and outputting.